Preheating apparatus for an extruder

ABSTRACT

An apparatus for heating and blending thermoplastic material prior to delivering the material into the barrel of an extruder has an elongated housing which contains two intermeshing feed screws that are driven to rotate in the same direction. Controlled heating is provided by electric band heaters which encircle the exterior of the housing, and electric cartridge heating elements fitted into aluminum cores that are received by an axial bore in each of the feed screws.

This is a continuation of application Ser. No. 08/008,014 filed on 22Jan. 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus for extruding thermoplasticmaterial. More particularly, the present invention relates to apparatusfor preheating thermoplastic material as it is blended and fed into thebarrel of an extruder.

2. Description of the Related Art

Plastics extruders generally comprise a cylindrical barrel within whichone or two plasticating feed screws are rotatably positioned. Theplastic material to be processed is introduced into the barrel near theend and is carried therethrough by the screw, which heats and softensthe material by physically working it. The heating imparted to theplastic by the mechanical working can be supplemented by externallyapplied heat as provided, for example, by band heaters applied to theouter circumference of the barrel. The material which issues through ashaping die at the opposite end of the barrel is generally of a fluid ormolten state of relatively high viscosity, yet suitable for forming intothe particular shapes desired.

In order to improve the output of processed plastic from an extruder, itis known in the art to preheat the thermoplastic material before itenters the extruder, thereby initially contributing some of the heatrequired for plastication. Since less heat is required to completeplastication, material is in the barrel for a shorter time and is thusprocessed more quickly. In addition, the preheater enables more stablecontrol of the temperature within the extruder by uniformly heating thematerial to ensure that it enters the barrel at a consistent preheattemperature.

Preheaters proposed by the prior art have typically used twocounter-rotating screws which generate substantial heat by mechanicallyworking the material as it is mixed and ultimately fed into theextruder. Supplemental heating has been provided either on an externalsurface of the preheater housing or internally of the screws by means ofa fluid to help attain the desired preheat temperature.

Although this approach has proven to be workable in many applications,it is unsatisfactory for processing thermoplastic materials with highlevels (greater than 20%) of mineral fillers, such as calcium carbonateor talc. Calcium carbonate, for example, is typically included invarious PVC compounds used to make plastic pipe. Preheaters areparticularly valuable when processing materials of this type since thefillers tend to inhibit heat conduction by the thermoplastic material asit passes through the extruder barrel. However, mechanically workingthis material with counter-rotating screws, as taught by the prior art,often creates too much heat due to the increased friction associatedwith the mineral filler; this causes inconsistencies in material flow tothe extruder, and can even overheat the material, making it unsuitablefor further processing.

Accordingly, it is an object of the present invention to overcome thedeficiencies in the prior art arrangements, and to provide an improvedpreheater for use with extruders which can effectively blend anduniformly heat thermoplastic materials having a high content of mineralfillers.

SUMMARY OF THE INVENTION

Briefly stated, in accordance with one aspect of the present invention,apparatus is provided for use on an extruder for preheating thethermoplastic material just before it enters the extruder barrel. Theapparatus includes an elongated housing, a pair of intermeshing feedscrews, means for driving the screws so that they rotate in the samedirection; and means for providing supplemental heating for the plasticmaterial as it passes through the preheater.

The use of co-rotating screws rather than counter-rotating screwsprovides a more uniform blending of the materials processed but does notwork the material in a way that generates significant heat during theprocess. Rather, heat is selectively added in a more controlled, uniformmanner by external band heaters and cartridge heaters within the screws,thus elevating the temperature of the material while it issimultaneously blended and mixed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a preheater mounted on an extruderin accordance with the present invention, with certain portions brokenaway for clarity.

FIG. 2 is a top plan view of the preheater as shown in FIG. 1 withcertain portions broken away and the hopper removed for clarity.

FIG. 3 is a plan view of the feed screw used in the preheater of thepresent invention.

FIG. 4 is an enlarged, fragmentary section view of the end of thepreheater as shown in FIG. 1.

FIG. 5 is an enlarged, fragmentary section view of an intermediateportion of the barrel of the preheater as shown in FIG. 1.

FIG. 6 is an enlarged, fragmentary section view of an intermediateportion of the preheater as shown in FIG. 2.

FIG. 7 is an enlarged section view of a portion of the preheater takenalong the line 7--7 shown in FIG. 6.

FIG. 8 is an enlarged, fragmentary section view of the distributorhousing of the preheater as shown in FIG. 1.

FIG. 9 is an enlarged, fragmentary section view of the distributorhousing of the preheater as shown in FIG. 2.

FIG. 10 is a fragmentary view taken along the line 10--10 of FIG. 9 androtated 90 degrees counterclockwise, diagrammatically illustrating thegear engagement for the drive mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 and FIG. 2, there is shown a preheater 10mounted on an extruder 12. More specifically, the attachment of thepreheater 10 to the extruder 12 is accomplished by a support frame 14attached to the gear box housing 16 of the extruder 12 by a number ofscrews 18. The specific configuration of the support frame 14 is notcritical to the present invention; rather, it is manufactured asrequired to facilitate connection of the preheater 10 to suitablemounting points, which will vary depending on the particularconfiguration of the extruder 12 (as defined by the manufacturer). Acylindrical swivel plate 20 extends below the preheater 10 and isreceived by a mating swivel plate retainer 22 affixed to the top of thesupport frame 14. It should be noted that the swivel plate 20 ispositioned on the preheater 10 at a point very near the center ofgravity to balance the weight and thus facilitate rotating the preheater10 on the swivel plate 20, as required, enabling easy access forcleaning or performing other service.

The preheater 10 also connects to the extruder 12 via a throat adapter24 which is attached to the underside of the preheater by means ofscrews 25. The throat adapter 24 mates to an cooling throat 26 on thebarrel 27 of the extruder 12 with a gasket (not shown), pivoting bolts30, and wing nuts 32. The throat adapter 24 and cooling throat 26 areboth hollow to provide a path for flow of thermoplastic material fromthe preheater 10 into the barrel 27 of the extruder 12, as will be morefully described in subsequent paragraphs.

The main body of the preheater 10 includes a barrel 34 that ismanufactured in two sections to facilitate fabrication; namely, an inletsection 36 and an outlet section 37. As shown by the detail in FIG. 5,the two sections 36,37 are joined together by split collars 40,41.Specifically, the mating ends of the inlet section 36 and outlet section37 are provided with annular grooves 38,39 which create shoulders 42,43on the respective barrel sections. The split collars 40,41 areconfigured to engage firmly the shoulders 42,43 when drawn tightlytogether by screws 44. This construction provides a rigid assembly forthe elongated barrel 34 as shown.

Referring now to FIG. 7, the barrel 34 is provided with an axial bore 46to receive two feed screws 50. The bore 46 actually comprises two,longitudinally intersecting cylindrical bores, the diameters of whichare just slightly greater than the outer diameter of the screws 50. Tocomplete the flow path through the barrel 34, there is an inlet opening47 through the top of the inlet section 36 which communicates with thebore 46, and an outlet opening 48 similarly joins bore 46 through thebottom of the outlet section 37.

The two feed screws 50 are identical in construction and have theconfiguration shown generally in FIG. 3. Specifically, each feed screw50 has a main body section 52, an open section 54, and a terminalsection 56. Both the main body section 52 and terminal section 56 areprovided with a helical flight forming a screw channel that is generallyU-shaped in cross section. Preferably, the flight of the screws 50 ismultiple start with a constant lead. Note also that the flight of themain body section 52 has a right-hand lead 53, while the terminalsection 56 has a left-hand lead 57. The open section 54 is cylindricalin cross-section, with the diameter being approximately equal to theroot diameter of the main body section 52 and the terminal section 56.Each screw 50 also has a cylindrical axial bore 58 and a spline 60machined on one end. Threaded into the bore 58 at the end opposite thespline 60, there is a screw tip 66 having multiple cylindrical sectionssized for various assembly purposes, as will be more fully described insubsequent paragraphs. The screw tip 66 also has an axial bore 67 whichis concentric with the bore 58 in the screws 50.

As best seen in FIG. 4, the barrel 34 is provided with an end plate 62on the outlet section 37 to support the ends of the screws 50.Specifically, bearings 64 are provided in the end plate 62 to receivethe cylindrical sections 65 of each screw tip 66 nearest the screws 50,thus providing a means of rotatably supporting the ends of the screws50. A bearing retainer 68 is attached with screws 73 to the end plate 62to hold the bearings 64 in place. It should be noted that the end plate62 is held in place by shoulder screws 70 which compress springs 72against the end plate 62 to provide the desired pressure on the screws50 which, in turn, bear against associated elements of the drivemechanism (as will be more fully described in subsequent paragraphs).This method of assembly maintains the desired clearance between theintermeshing flights by supporting the screws 50 at each end whileallowing for differential thermal expansion between the screws 50 andbarrel 34. Finally, annular sealing elements 74 are provided at thelocations shown to prevent the bearing 64 from being contaminated withparticles of thermoplastic material that are used in conjunction withthe preheater 10.

The opposite end of the barrel 34 (the inlet section 36) is similarlyclosed at distributor housing 76 (see FIG. 8). A split collar 78 ispartially received in an annular groove 79 and engages a shoulder 81created by the groove 79 in the end of the barrel inlet section 36,thereby connecting the barrel 34 to an end wall 83 of the distributorhousing 76 when assembled with screws 80. The splines 60 of the feedscrews 50 extend into the distributor housing 76 and are piloted intodriven gears 85,86 which are supported on one end by needle bearings 82in the wall 83 of the distributor housing 76 and at the opposite end bybearings 93 in the housing end cap 77. Proper alignment of the barrel 34with the distributor housing 76 is facilitated by the use of pins 84received by appropriately located bores in the adjacent components.

As seen in FIGS. 8-10, the spline 60 of each screw 50 is received withina mating spline 88,89 on the inner diameter of driven gears 85,86.Intermeshing with the driven gears 85,86 is a drive gear 87. The drivengears 85,86 are dimensionally identical; however, the mating spline 88of gear 85 is offset by an angle A with respect to a reference positionR. The effect of this offset A is to establish a different alignment ofgear 85 with the helical flight of the associated screw 50, as comparedto gear 86 on the adjacent screw. The differing alignments of the drivengears 85,86 enables the proper timing of rotation so that the helicalflights of the adjacent screws 50 will intermesh properly, see FIG. 6.

Rotational movement is imparted to drive gear 87 by a motor 90 andgearbox 91 (see FIGS. 1 and 2). Preferably, the motor 90 is providedwith an attached blower assembly 99 to supply a cooling air flow duringoperation. Referring back to FIGS. 8, 9 and 10, the central shaft 92 ofdrive gear 87 is rotatably supported by bearings 95 fitted at one end inthe wall 83 of distributor housing 76, and at the other end in housingend cap 77. As indicated by the arrows in FIG. 10, the drive gear 87 iscaused to rotate by the motor 90 through gearbox 91 in acounterclockwise manner, thus causing the driven gears 85,86 to rotatein a clockwise direction and thereby impart a corresponding rotation tothe screws 50. As should now be apparent, this gearing arrangementprovides a common drive for the screw 50 and maintains the relativetiming of the screws 50 to prevent interference between flights.

Referring again to FIGS. 1 and 2, defining the path for thermoplasticmaterial to flow into the preheater 10 is an inlet cooling throat 94mounted in line with the opening 47 in the inlet section 36 of thebarrel 34. The cooling throat 94 prevents conductive heat transfer fromthe barrel 34 to this portion of the flow path. If the thermoplasticmaterial is heated before it enters the barrel 34, it can become"tacky", causing erratic material flow or, possibly, creating anobstruction in the flow path. A mounting plate 96 adapts the coolingthroat 94 for attachment of a drawer magnet 98. The magnet 98 isdesigned to catch any particles of ferrous metals that may beinadvertently mixed with the thermoplastic material, thus preventingsuch particles from causing damage to the screws and barrel of preheater10 or extruder 12. A hopper 100 is mounted atop the drawer magnet 98 toprovide a suitable material reservoir.

As stated previously, the function of the preheater 10 is to deliverthoroughly blended plastic to the extruder 12 at a consistent, elevatedtemperature. As material is blended by the rotating screws 50, somefrictional heat is added; however, the primary heating capabilities ofthe preheater 10 are provided by two other sources that can be moreaccurately monitored and controlled. First, there is a series of bandheaters 102 encircling the outer surface of the barrel 34. The heaters102 are electrical resistance units with suitable wire leads 103 tofacilitate wiring at assembly, as are typically used in the art for thistype of application.

The second source of supplemental heating comes from within aluminumheater cores 104 that are inserted into the axial bores 58 of each feedscrew 50. Specifically, as shown in FIGS. 6 and 7, the heater cores 104are elongated aluminum cylinders with three equally spaced longitudinalbores to receive cartridge heating elements 106. The cores 104 thusprovide a means to dissipate uniformly the heat supplied by heatingelements 106 within the bore 58 of the screws 50.

Since the heating elements 106 are preferably electric resistance, rodtype, power is supplied via slip ring assemblies 108 (see FIG. 4). Morespecifically, power is supplied to each brush block assembly 114 whichis held in place by a post and plate assembly 116 bolted to the bearingretainer 68. Spring loaded brushes 118 transfer electrical current tocollector ring assemblies 120 that are attached to threaded sections 69of the screw tips 66. This construction provides rotary contacts tosupply power to the cartridge heaters 106 through electrical wiring (notshown) which connects the heaters 106 to terminal posts 122 on thecollector ring assemblies 120, the bores 67 in screw tips 66 providingconduits for passage of the electrical wiring. A cover 110 is attachedwith screws 111 to end plate 62 to prevent the contact surfaces of theslip ring assemblies 108 from becoming contaminated and adverselyaffecting their ability to conduct electricity. To facilitate accuratetemperature control, thermocouples 112 are provided in the barrel 34 andin the heater cores 104.

In operation, thermoplastic material is gravity fed into the preheater10 from the hopper 100, passing through the drawer magnet 98, throughthe cooling throat 94, and into the opening 47 of inlet section 36 ofthe barrel 34. When the material enters the barrel 34, it is depositedon the feed screws 50 which are both rotating clockwise (as viewed fromthe exit end of the preheater 10). The rotation of the intermeshingscrews 50 serves to blend the material and simultaneously advance ittoward the outlet 48 of outlet section 37 of the barrel 34. As thematerial moves through the barrel 34 from the inlet 47 toward the outlet48, it is heated by conduction from the inner surface of the barrel 34which is first heated by the band heaters 102, and conduction from thesurface of the feed screws 50 which are heated internally by thecartridge heating elements 106. The material temperature is closelycontrolled by regulating the power to heating elements 102 and 106, asnecessary, to achieve the desired respective temperatures in the barrel34 and screws 50, as indicated by thermocouples 112. As the materialreaches the open section 54 of the screws 50, internal pressure causesit to exit the preheater 10 through the outlet 48 of the barrel 34. Itshould be noted that the right-hand lead 53 on the main body 52 of thescrews 50 and the left-hand lead 57 on the terminal section 56concurrently act to advance material toward the feed section 54,creating the internal pressure which acts to move the material throughthe outlet 48. The thermoplastic material then passes on through thethroat adapter 24, through the cooling throat 26 and into the extruderbarrel 27.

The material leaving the preheater 10 and entering the extruder 12 hasbeen thoroughly blended and heated to an optimal temperature for furtherprocessing. It can thus be seen that the present invention providesdistinct advantages over the preheaters disclosed in the prior art, inthat it delivers a consistent flow of thermoplastic material directly tothe extruder barrel, thereby enabling maximum production rates.

Although particular embodiments of the present invention have beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications can be made without departingfrom the spirit of the present invention. For example, instead ofelectric cartridge heaters, a temperature controlled fluid could becirculated, as desired, through the feed screws to provide supplementalheating. Alternatively, various flight configurations for the feedscrews can effectively blend and advance the material. It is thereforeintended to encompass within the appended claims all such changes andmodifications that fall within the scope of the present invention.

What is claimed is:
 1. An apparatus for preheating thermoplasticmaterial prior to entry of the material into an extruder barrel,comprising:an elongated housing having a central bore, an inlet adjacenta first end of the housing, and an outlet between the inlet and a secondend opposite the first end of the housing, both the inlet and the outletintersecting the central bore; a material flow path beginning at theinlet, continuing through the central bore, and passing from the centralbore through the outlet; a pair of adjacent, parallel feed screwsreceived within the central bore of the housing for blending thethermoplastic material and advancing it along the material flow path,each feed screw having a main body section having a helical flightconfigured to form a screw channel that is generally U-shared incross-section and to advance the material from the inlet toward theoutlet when the feed screw is rotated, an open section having a constantdiameter, and a terminal section having a helical flight configured toform a screw channel that is generally U-shaped in cross-section andwith a lead opposite that of the main body section, such that the mainbody section extends from the inlet to the open section which isadjacent the outlet, and the terminal section extends from the opensection to the second end of the housing; drive means for rotating thefeed screws in the same direction within the central bore; and means forheating the thermoplastic material to a desired temperature as it isblended and advanced along the material flow path.
 2. The apparatus ofclaim 1 wherein the feed screws are positioned within the elongatedhousing such that the helical flights of the adjacent screws intermesh.3. The apparatus of claim 2 wherein the drive means for rotating thefeed screws in the same direction includes a driven gear non-rotatablyfitted on each of the feed screws, and a drive gear that engages both ofthe driven gears.
 4. The apparatus of claim 1 wherein the open sectionof each of the feed screws has a cylindrical configuration with adiameter approximately equal to the root diameter of the screw.
 5. Theapparatus of claim 1 wherein the material heating means includes meansdisposed within the feed screws for directly heating the feed screws. 6.The apparatus of claim 5 wherein the means for heating the feed screwsincludes a cylindrical heater core that is received by an axial bore ineach of the feed screws, the heater core having at least onelongitudinal bore to receive an electric, rod-like heating element. 7.The apparatus of claim 5 wherein the material heating means furtherincludes electric band heaters mounted on the exterior of the housing.